The invention relates to a method of operating an internal combustion engine, in which a high-pressure pump pumps fuel into a pressure accumulator, a delivery quantity is supplied to the high-pressure pump using a metering unit, and an injected fuel quantity is withdrawn from the pressure accumulator and then injected. In addition, the invention relates to an internal combustion engine suitable for implementing this method.
A fuel delivery system, in which the fuel is delivered by a high-pressure pump into a pressure accumulator used to jointly supply a plurality of injectors, is referred to as a common-rail system.
The withdrawal of a quantity of fuel from the pressure accumulator used for injection (i.e., the injected fuel quantity) and/or leakage and control quantities of the injectors may result in a reduction of the fuel pressure in the pressure accumulator of a common-rail system.
The quantity of fuel identified as delivery quantity, which is supplied to a suction side of the high-pressure pump, should be distinguished from the injected fuel quantity. The delivery quantity is the injected fuel quantity plus the leakage and control quantities of the injectors.
Normally, a pressure control valve assigned to the pressure accumulator regulates the fuel pressure in the pressure accumulator of common-rail systems by returning a quantity of fuel to the fuel tank if the quantity of fuel exceeds a quantity required to attain or maintain the fuel pressure in the pressure accumulator.
In addition to pressure regulation by a pressure control valve, common-rail systems may regulate the quantity of the fuel supplied to the high-pressure pump.
In this connection, a metering unit may limit the delivery quantity supplied to the high-pressure pump to a value momentarily needed to maintain/attain a specified desired pressure in the pressure accumulator.
The suction side quantity regulation may avoid an unnecessary compression of surplus fuel by the high-pressure pump and a following decompression by the pressure control valve, which may contribute to a reduction of the power consumed by the fuel injection system, as wells as the temperature of the fuel in the system.
It is believed that a disadvantage of suction-side quantity regulation is that the system may be unable to optimally react to rapid changes in the injected fuel quantity, with respect to pressure regulation in the pressure accumulator.
Subsequent to a rapid change in the injected fuel quantity after a delay, the metering unit may deliver a delivery quantity adjusted to the new injected fuel quantity to the high-pressure pump, which may include multiple pistons.
However, a pump piston may have completed its suction stroke shortly before the change in the injected fuel quantity and thus may still been charged with an old delivery quantity corresponding to the old injected fuel quantity. This old delivery quantity may still be supplied to the pressure accumulator in the next discharge stroke of the pump piston.
The quantity difference between the old delivery quantity, which is still delivered into the pressure accumulator, and the new injected fuel quantity already withdrawn from the pressure accumulator may result in pressure changes in the pressure accumulator. It is believed that these pressure differences are directly related to the quantity difference.
If the injected fuel quantity abruptly increases, for example, if more fuel is withdrawn from the pressure accumulator by the immediate injection of the new, larger injected fuel quantity than is deliverable by the subsequent discharge stroke, which delivers only the old delivery quantity, the fuel pressure in the pressure accumulator may drop.
A sudden reduction in the injected fuel quantity may be more critical. Less fuel is withdrawn for injection from the pressure accumulator than is supplied to the pressure accumulator by a following piston stroke. This may result in a pressure rise in the pressure accumulator, which may reduce the service life of both the pressure accumulator and the high-pressure components connected to it.
An object of the present invention is to provide a method of operating an internal combustion engine, in which the fuel pressure in a pressure accumulator of the internal combustion engine is stabilized, to reduce the pressure load on components of the fuel delivery system.
In accordance with an exemplary embodiment of the present invention, an adjustment of the delivery quantity only effects the fuel pressure in the pressure accumulator when a first pump piston of the high-pressure pump is charged with the new delivery quantity in its suction stroke and this pump piston starts its discharge stroke into the pressure accumulator.
The time between the change in the injected fuel quantity and the first-time delivery of a new delivery quantity into the pressure accumulator is identified as dead time and is essentially a function of the time lag of the metering unit, the condition of the high-pressure pump at the time of the change in the injected fuel quantity, as well as the geometry of the high-pressure pump. In addition, the dead time is a function of the speed of the high-pressure pump in relation to the speed of the internal combustion engine.
An exemplary method of operating an internal combustion engine according to the present invention, in which a high-pressure pump pumps fuel into a pressure accumulator, a delivery quantity is supplied to the high-pressure pump using a metering unit and an injected fuel quantity is withdrawn from the pressure accumulator and injected, is characterized in that the delivery quantity is changed as a function of the new value of the injected fuel quantity as soon as a change in the injected fuel quantity from an old value to a new value is provided, the old injected fuel quantity continuing to be injected for a selectable delay time.
It is believed to be advantageous to establish a threshold value for the change in the injected fuel quantity and to change the delivery quantity only after it is exceeded.
In another exemplary method according to the present invention, the new injected fuel quantity is injected at the end of the above-described selectable delay time.
It is believed to be advantageous to select the delay time to at least roughly correspond to the dead time of the high-pressure pump.
The high-pressure pump may be mechanically driven, for example, by the internal combustion engine, a gear unit being interconnected to adjust the speed, depending on the type of internal combustion engine.
When driven by the internal combustion engine, the dead time of the high-pressure pump may be related to a crankshaft angle of the internal combustion engine, using the gear ratio between the high-pressure pump and the engine, as well as the number of pump pistons. The number of cylinders may be used to relate the dead time to the number of injections, which may be useful in specifying the delay time, since the internal combustion engine""s dependence on the speed is eliminated.
The proportion of one pump stroke to supplying the injectors is determined by the number of cylinders and the gear ratio. The proportion of the fuel required for the next injection, which is located in the pump cylinder, may thus be known. As a function of these two parameters, the delay time or the number of injections corresponding to the delay time may be defined.
Furthermore, the delay time may be selected as a function of the operating state and/or the load of the internal combustion engine.
The high-pressure pump may be driven by a drive that is separate from the internal combustion engine, both the speed of the high-pressure pump and the speed of the internal combustion engine having to be considered in determining the delay time.
Injecting the old injected fuel quantity during the delay time withdraws the quantity of fuel from the pressure accumulator, which is delivered to the pressure accumulator by those pistons of the high-pressure pump that have completed their suction stroke before the change in the injected fuel quantity. Thus, these pistons deliver an old delivery quantity into the pressure accumulator that corresponds to the old injected fuel quantity.
In this manner, the fuel pressure in the pressure accumulator is stabilized, even when injected fuel quantities abruptly change, for example, as the result of abrupt changes in pedal travel when changing gears.
For example, a pressure increase in the pressure accumulator may be avoided when the injected fuel quantity is reduced as a result of a quantity of fuel corresponding to the old injected fuel quantity being delivered to the pressure accumulator during the dead time of the pump, while, however, only the reduced, new injected fuel quantity is withdrawn from the pressure accumulator. The pressure load on the high-pressure pump, the pressure accumulator, and additional components of the fuel injection system may thus be reduced and their service life may be increased.
Moreover, an exemplary method according to the present invention may avoid a reduction in pressure in the pressure accumulator, if a greater injected fuel quantity is to be injected. It is believed that the reduction in pressure results from more fuel being withdrawn from the pressure accumulator by injection than it is possible for the high-pressure pump to deliver to the pressure accumulator during its dead time.
According to an exemplary embodiment of the present invention, the retention of the quantity of fuel injected before the change in the injected fuel quantity for a selectable delay time may permit the fuel pressure in the pressure accumulator to remain constant, until a quantity of fuel that corresponds to the new injected fuel quantity may be delivered.
It is believed that an additional advantage of an exemplary method according to the present invention is that the response time to changes in the injected fuel quantity and a resulting adjustment of the delivery quantity is very short compared to other methods based, for example, on the filtering of quantity signals.
Since an exemplary method according to the present invention already considers the dead time of the high-pressure pump, processing in the pressure regulator may be superfluous.
It is believed that another advantage of an exemplary method according to the present invention is the low amount of calculation believed to be required, since only old and already determined values should be retained for the injected fuel quantity.
Selecting the delay time as a function of the speed of the internal combustion engine, as described above, may allow the selection of a delay time greater than 0, if the speed of the internal combustion engine exceeds a specified minimum.
An exemplary method according to the present invention may be implemented as a computer program provided for a control unit of an internal combustion engine, for example, an internal combustion engine of a motor vehicle. In this regard, the computer program may execute on a microprocessor, thereby implementing an exemplary method according to the present invention. The computer program may be stored, for example, in an electric memory medium, for example, a flash memory or a read-only memory.